The study evaluates the safety and efficacy of 1592U89 (abacavir) in patients with HIV dementia. The study is a randomized, double-blind, placebo-controlled multicenter study, conducted at several centers in the United States, United Kingdom, Holland, and Australia. The study enrolled approximately 90 patients, The double-blind, placebo controlled phase of the study is 12 weeks. After this phase, patients are eligible to receive 40 weeks of open label 1592U89 600 mg every 12 hours followed by the option to extend open-label 1592U89 indefinitely. Screening, baseline, and maintenance evaluations include: medical and neurological history including AIDS defining diseases, physical examination, neurological and neuropsychological assessments, dementia severity rating, concomitant medication assessment, head MRI, laboratory tests including routine safety labs (plasma and urinalysis), pregnancy screen, plasma virology and lumbar puncture for routine CSF analysis, cultures, immunological markers, and virology.